The present invention relates to a pipe stalk and a method for assembling a pipe-in-pipe pipeline from a plurality of such pipe stalks, and in particular to a method and apparatus for assembling a pipe-in-pipe pipleline directly on a pipe take-off assembly of a pipe laying vessel.
Marine pipelines for conveying gas or crude oil are typically laid from specialised pipelaying vessels or barges, whereby pipelines are deployed and lowered to the seabed in a controlled fashion, the pipelines either being assembled from individual pipe sections on the vessel (referred to as “stovepiping”) or spooled from one or more reels mounted on the vessel (referred to as “reel pipelaying”).
The stovepiping method generally requires the pipelaying operation to be periodically halted to permit additional pipe sections to be welded to the pipeline and also requires many experienced welders and additional welding equipment to be carried on the vessel, working in difficult conditions. Therefore stovepiping is a relatively slow and difficult process.
By contrast, reel pipelaying, wherein the pipeline can be prefabricated onshore and spooled onto a large reel, reduces the labour requirements onboard the vessel and considerably speeds up the pipelaying operation. However, a requirement of the reel pipelaying method is to provide means for straightening and guiding the pipeline as it leaves the reel, typically by means of a series of rollers or tracks that impart sufficient reverse bending force to residual curvature from the pipeline and guides the pipeline into the water at an angle optimised to reduce bending stresses.
The resistance to flow of liquid products such as oil increases as temperature decreases. Therefore it has become common to use double walled pipelines for transporting crude oil and gas from subsea locations, where the pipeline is formed from an inner pipe carrying the fluid to be conveyed, and a concentric outer pipe, the annular space between the inner and outer pipes being filled with a thermally insulating material to insulate the inner pipe. Such arrangements are known as “pipe-in-pipe” installations. A problem with such known arrangements is the need to perform two separate welding operations when joining two lengths of pipeline or individual pipe stalks together. To gain access to the inner pipelines it is often necessary to remove sections of the outer pipeline and then provide means for creating a joint between the ends of the outer pipeline once the inner pipeline has been welded.
Typically the outer pipes of a pipe-in-pipe flowline are joined together using two half shells required to bridge the gap between the ends of the outer pipes. This requires cutting of the pipe ends to ensure sufficient room for the half shells and several welding processes for joining the half shells to the ends of the adjacent outer pipes and to each other, leading to longer installation times and a greater risk of leakage. Additionally it is necessary to store the half shells on the installation vessel along with the pipe stalks, requiring greater storage space on the vessel and limiting the amount of pipe that can be laid.